Much of our general world knowledge is about the properties and attributes of objects. Frequently, these properties constitute a location on an underlying continuum that is "gradable" (e.g., cold, hot, etc.). The primary aim of an earlier proposal was to document similar effects of context on categorical judgments (e.g., "whales are large") and relational judgments (e.g., "whales are larger than fleas"). The focus of the current proposal is to develop a formal model of these uses of property knowledge and to extend this account to the effects of semantic organization on these judgments. This proposal assumes that gradable properties are evaluated with respect to norms associated with the current context. In this conceptualization, the current context is described as a "reference class" and the norms are described as "reference points" located at the most extreme values associated with the reference class. According to the proposed model of comparative judgment, relative magnitude is determined by comparing the distances of each alternative to a common reference point. These mechanisms are extended to categorical judgments by assuming that these judgments are made by comparing the distance between an object and a reference point to a criterion. Additional experiments are proposed to test the hypothesis that conceptual categories play an important role in establishing reference classes. An understanding of these fundamental issues in semantic memory is crucial to a determination of the exact nature of memory impairments in certain forms of dementia. For example, Alzheimer patients appear to have impaired semantic memory function that is absent in the normal old (Nebes, 1989). Currently, it is not known whether these impairments stem primarily from a loss of semantic information, reduced accessibility of this information, or subsequent limitations on its use. A resolution of these issues is unlikely in the absence of adequate theories of semantic memory.